Signal transduction collectively encompasses the diverse biochemical reactions involved in modulating programmed cellular responses to external and internal stimuli. Among the cellular enzymes involved in this complex process, the tyrosine protein kinases appear to play key roles in mediating signaling events that aid in the regulation of cell growth and differentiation. We have shown that several members of the src family of non-receptor tyrosine protein kinase protooncogenes function as signaling components of diverse receptors in cells of hematopoietic origin.